Semiconductor device having a spacer layer doped with slower diffusing atoms than substrate

ABSTRACT

A semiconductor device includes a silicon substrate heavily-doped with phosphorous. A spacer layer is disposed over the substrate and is doped with dopant atoms having a diffusion coefficient in the spacer layer material that is less than the diffusion coefficient of phosphorous in silicon. An epitaxial layer is also disposed over the substrate. A device layer is disposed over the substrate, and over the epitaxial and spacer layers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. utility patent application Ser. No.10/851,764 filed May 21, 2004.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor devices and,more particularly, to the epitaxial layer of semiconductor devices.

DESCRIPTION OF THE RELATED ART

The resistivity of silicon wafers heavily doped with phosphorous (P) canbe less than one one-thousandth (0.001) ohms-centimeter (ohms-cm).Substrates with such low resistivity are desirable for many reasons,including the ability to form thereon devices having improved conductionor reduced on-state resistance.

Devices fabricated from heavily phosphorous-doped silicon wafers mayrequire measures to prevent the diffusion of phosphorous atoms from thesubstrate into the active or device region that can occur duringfabrication processes that are carried out at elevated temperatures,such as, for example, annealing. Increasing the thickness of the deviceepitaxial layer, or adding an epitaxial layer/spacer to the device, isone measure that can be used to reduce the effects of such diffusion.The use of a thicker or additional epitaxial layer/spacer increases theseparation between the substrate and the device/active region, andthereby reduces the impact of the diffusion of phosphorous. However, thehigher resistivity of a thicker or additional epitaxial layer (relativeto the substrate) significantly and undesirably increases the on-stateresistance of the device.

Therefore, what is needed in the art is a semiconductor device having astructure that reduces diffusion of phosphorous atoms into thedevice/active region from a substrate that is heavily-doped withphosphorous and yet maintains a relatively low on-state resistance, anda method of forming same.

Furthermore, what is needed in the art is a semiconductor device havinga relatively thick epitaxial layer with a relatively low or reducedresistivity and which reduces diffusion of phosphorous atoms from asubstrate that is heavily-doped with phosphorous into the device/activeregion, and a method of forming same.

SUMMARY OF THE INVENTION

The present invention provides a semiconductor device and structure thatsubstantially reduces diffusion of phosphorous atoms from aheavily-doped phosphorous substrate to the device active layer.

The invention comprises, in one form thereof, a semiconductor devicehaving a silicon substrate heavily-doped with phosphorous. A spacerlayer is disposed over the substrate and is doped with dopant atomshaving a diffusion coefficient in the spacer layer material that is lessthan the diffusion coefficient of phosphorous in silicon. An epitaxiallayer is also disposed over the substrate. A device layer is disposedover the substrate, and over the epitaxial and spacer layers.

An advantage of the present invention is that diffusion of phosphorousatoms from the heavily-doped phosphorous substrate to the device activelayer is substantially reduced.

Another advantage of the present invention is that the impact of thespacer-layer on the device on-state resistance is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of this invention,and the manner of attaining them, will become apparent and be betterunderstood by reference to the following description of one embodimentof the invention in conjunction with the accompanying drawings, wherein:

FIG. 1 plots the drain-to-source resistance (R_(DSon)) and thewafer-level breakdown voltage (BVDSS) as a function of the thickness ofthe epitaxial spacer layer of a device formed on a substrate heavilydoped with phosphorous atoms;

FIG. 2 is a cross-sectional view of a portion of one embodiment of asemiconductor device of the present invention;

FIG. 3 plots the diffusion coefficients of phosphorous, arsenic andantimony in silicon versus temperature,

FIG. 4 plots the resistivity profiles of three different epitaxialstructures formed on phosphorous-doped substrates versus thickness ofthe epitaxial structures; and

FIG. 5 shows a flow diagram of one embodiment of a method of fabricatinga semiconductor device of the present invention.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate one preferred embodiment of the invention, in one form, andsuch exemplifications are not to be construed as limiting the scope ofthe invention in any manner.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now to the drawings and particularly to FIG. 1, thedrain-to-source resistance (R_(DSon)) and the wafer-level breakdownvoltage (BVDSS) are plotted as a function of the thickness of theepitaxial layer or spacer of a device formed on a substrate heavilydoped with phosphorous atoms. The plot of R_(DSon) corresponds to anapplied gate voltage of 10 Volts. The resistivity of the epitaxial layeror spacer is the same as the resistivity of the active/device region.The dashed line in FIG. 1 represents typical levels of R_(DSon) andBVDSS for a device formed on an arsenic-doped substrate but otherwiseidentical to the device formed on the phosphorous doped substrate.

As is apparent from FIG. 17 both R_(DSon) and BVDSS increase rapidlywith the thickness of the epitaxial layer/spacer in the device formed onthe phosphorous-doped substrate. As is also apparent from FIG. 1, aphosphorous-doped substrate requires increasing the thickness of theepitaxial layer by, or adding an epitaxial spacer layer having athickness of, approximately 2 microns (μ) in order to achieve a BVDSSthat is equivalent to the BVDSS of the device formed on thearsenic-doped substrate as represented by the dashed line. However,increasing the thickness of the epitaxial layer by, or adding anepitaxial spacer having a thickness of, approximately 2 microns (μ)increases R_(DSon) by approximately thirty percent relative to a devicehaving an epitaxial layer/spacer approximately 1.2μ thick. It isdesirable to achieve the reduction in diffusion of phosphorous atomsthat is achieved by increasing the thickness of the epitaxiallayer/spacer and yet avoid the increase in R_(DSon).

Referring now to FIG. 2, one embodiment of a semiconductor device of thepresent invention is shown. Semiconductor device 10, such as, forexample, an N-type metal oxide semiconductor field effect transistor(MOSFET), includes substrate 12, epitaxial spacer 14, and epitaxiallayer 16. Within epitaxial layer 16 is formed device/active layer 20 anddrain region 22. Device 10 also includes various features andstructures, such as, for example, trenches, gates, sources, etc, formedwithin device layer 20. The particular embodiment of device 10 shownincludes P− well regions 24, P+ body regions 26, N+ source regions 28,and gate trench 30 which is lined with gate dielectric material 32 andis filled with gate conductive material 34. Interlevel dielectric layer36 is deposited over gate trench 30 and partially over sources 28, andmetal layer 38 is also deposited, by procedures and for purposes wellknown in the art.

Substrate 12 is an N+ silicon substrate that is heavily N+ doped withphosphorous atoms P. Substrate 12 is doped to a dopant concentration of,for example, greater than 5E18 phosphorous atoms/cm⁻³.

Epitaxial spacer 14 is a layer of N-type silicon having a thickness Tthat is formed or epitaxially grown over substrate 12. Thickness T ofepitaxial spacer 14 is dependent at least in part upon the total thermalbudget of the particular technology family to which device 10 belongs orin which device 10 is classified. Semiconductor devices within atechnology family are generally exposed to common fabrication processes,some of which occur or are carried out at elevated temperatures.Elevated temperatures facilitate diffusion of the phosphorous dopantatoms from within substrate 12 to the active/device region 20. Thus,thickness T of first epitaxial spacer 14 is increased or relativelylarge when device 10 has a relatively high thermal budget, i.e., is tobe exposed to a relatively large number of elevated-temperaturefabrication processes that encourage diffusion of the phosphorous atomsfrom substrate 10 to active/device layer 20. Conversely, thickness T ofepitaxial spacer 14 is reduced or relatively small when device 10 has asmaller thermal budget and therefore is to be exposed to fewerdiffusion-generating elevated temperature processes.

Epitaxial spacer 14 is doped, such as, for example, in situ, by ionimplantation, or by other suitable processes, with one or more types ofatoms A having diffusion coefficients that are a predetermined amountless than the diffusion coefficient of the phosphorous atoms P withwhich silicon substrate 12 is doped. Atoms A include, for example,arsenic, antimony, and other suitable dopant atoms. Epitaxial spacer 14is doped to a dopant concentration that is from approximately two toapproximately twenty times the dopant concentration of drain region 22and/or epitaxial layer 16, which are typically doped to a concentrationof from approximately 1E14 to approximately 1E17 phosphorous atoms/cm⁻³.Thus, epitaxial spacer 14 is doped from 2E14 to approximately 20E17phosphorous atoms/cm⁻³.

The specific dopant concentration to which epitaxial spacer 14 is dopedis also generally dependent at least in part upon the thickness ofspacer 14. More particularly, as the thickness of epitaxial spacer 14increases the concentration to which epitaxial spacer 14 is doped mustalso be increased. Increasing the concentration to which epitaxialspacer 14 is doped as thickness T increases enables a given or desired(relatively small) value of R_(DSon) to be achieved or maintained byreducing the per-unit resistivity of epitaxial spacer 14. Conversely, agiven or desired value of R_(DSon) is achieved or maintained asthickness T decreases by reducing the dopant concentration to whichepitaxial spacer 14 is doped. The reduced thickness offsets the increasein per-unit resistivity resulting from a reduced dopant concentrationwithin spacer 14.

As is discussed above, epitaxial spacer 14 is doped with one or moretypes of atoms having diffusion coefficients that are a predeterminedamount less than the diffusion coefficient of the phosphorous atoms withwhich the silicon substrate is doped. Examples of atoms having smallerdiffusion coefficients than phosphorous include arsenic and antimony.FIG. 3 plots the diffusion coefficients of phosphorous (curve P),arsenic (curve As) and antimony (curve Sb) in silicon versustemperature, and shows that the diffusion coefficients of arsenic andantimony are approximately 100 times smaller (i.e., slower) thanphosphorous. Doping epitaxial spacer 14 with atoms having slowerdiffusion rates than the phosphorous atoms in substrate 12 enables theconcentration to which epitaxial spacer 14 is doped to be significantlyincreased which correspondingly and significantly reduces the resistancethereof and thereby maintains a desirably low value of R_(DSon) fordevice 10.

It should be particularly noted that epitaxial spacer 14 can be doped toa level that is approximately the same concentration level to which aconventional arsenic-doped substrate is doped. Thus, the on-resistanceof epitaxial spacer 14 is substantially less than the on-resistance of aconventional epitaxial layer of the same thickness. Further, thethicknesses of the epitaxial layers in most conventional devices areoptimized for an arsenic-doped substrate. The embodiment of the presentinvention wherein epitaxial spacer 14 is doped with arsenic atoms to aconcentration level similar to that to which conventional arsenicsubstrates are doped, does not significantly affect the BVDSS of device10. In effect, the arsenic-doped epitaxial spacer 14 acts as a virtualarsenic substrate to device 10 so long as thickness T is sufficient toprevent diffusion of the phosphorous atoms into the device/active region20. Similar benefits are obtained with an antimony-doped epitaxialspacer 14 due to the close similarity of the diffusion coefficients ofthe two dopants in silicon, as discussed above and shown in FIG. 3.

Referring now to FIG. 4, standard resistivity profiles of threedifferent epitaxial structures formed on phosphorous-doped substratesare plotted versus thickness after all thermal processes have beencarried out for a typical advanced trench technology device. Thesubstrates have a resistivity of approximately 0.0013 ohm-cm.Resistivity profile 50 corresponds to a structure including an intrinsic(undoped) spacer layer having a thickness of 1.5μ, and an epitaxiallayer with a dopant concentration of approximately 3E16 atoms-cm⁻³(i.e., a resistivity of approximately 0.215 ohm-cm) with a thickness ofapproximately 4μ. Resistivity profile 60 corresponds to a structureincluding an intrinsic layer having a thickness of 0.5μ, a spacer layerhaving a thickness of approximately 1.0μ with a dopant concentration ofapproximately 3E16 atoms-cm⁻³ (i.e., a resistivity of approximately0.215 ohm-cm), and an epitaxial layer having a thickness ofapproximately 4μ also with a dopant concentration of approximately 3E16atoms-cm⁻³ (i.e., a resistivity of approximately 0.215 ohm-cm). Inshort, resistivity profile 50 is for a device or substrate having anundoped spacer layer whereas resistivity profile 60 is for a device orsubstrate having a spacer layer 14 with an increased dopantconcentration (and thus a lower resistivity). The resistivity profiles50 and 60 are virtually identical, and thereby show that the dopantconcentration of the spacer layer does not impact upper diffusion. Thus,in light of the slower diffusion rates of arsenic atoms in silicon, thedopant concentration of the spacer layer is increased thereby reducingits resistance and, in turn, reducing R_(DSon) of device 10.

Referring now to FIG. 5, one embodiment of a method of fabricating asemiconductor device of the present invention is shown. Method 100includes the steps of forming a spacer layer 102, doping the spacerlayer 104, forming an epitaxial layer 106, and other processing 108.

The process of forming spacer layer 102 includes forming, such as, forexample, by epitaxial growth, a spacer layer of a predetermined ordesired thickness over a phosphorous-doped silicon substrate. Theprocess of doping spacer layer 104 includes doping the spacer layer withdopant atoms that diffuse in silicon more slowly than phosphorous atoms,such as, for example arsenic and/or antimony or any other suitabledopant atoms. The process of doping spacer layer 104 may be integraland/or be simultaneous with the process of forming spacer layer 102. Theprocess of forming device epitaxial layer 106 includes forming, such as,for example, by epitaxial growth, a device epitaxial layer having apredetermined or desired thickness over the spacer layer. Otherprocessing 108 includes standard and conventional substrate andintegrated circuit fabrication processes known in the art.

In the embodiment shown, device 10 is depicted as a trench-gated MOSFET.However, it is to be understood that the device of the present inventioncan be alternately configured, such as, for example, a trench device ofa different technology family and/or a planar device.

While this invention has been described as having a preferred design,the present invention can be further modified within the spirit andscope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the present inventionusing the general principles disclosed herein. Further, this applicationis intended to cover such departures from the present disclosure as comewithin the known or customary practice in the art to which thisinvention pertains and which fall within the limits of the appendedclaims.

1. A method of fabricating a semiconductor device on a siliconsubstrate, said silicon substrate heavily-doped with phosphorous atoms,said method comprising: forming a spacer layer over said substrate;doping said spacer layer with dopant atoms having a diffusioncoefficient in said spacer layer that is less than a diffusioncoefficient of phosphorous in silicon; forming an epitaxial layer oversaid substrate; and forming an active layer over said substrate.
 2. Themethod of claim 1, wherein said dopant atoms include at least one ofarsenic and antimony.
 3. The method of claim 1, wherein said forming anddoping processes occur in a substantially simultaneous manner.
 4. Themethod of claim 1, wherein said forming a spacer layer comprisesepitaxially grown silicon.
 5. The method of claim 1, wherein said spacerlayer is formed directly upon said substrate, and said epitaxial layeris disposed over said spacer layer, and said active area is disposedover said epitaxial layer.
 6. A method of reducing diffusion ofphosphorous atoms from a silicon substrate heavily-doped withphosphorous atoms to a device layer of a semiconductor device, saidmethod comprising: forming a spacer layer over said substrate; anddoping said spacer layer with dopant atoms having a diffusioncoefficient in said spacer layer that is less than a diffusioncoefficient of phosphorous in silicon.
 7. The method of reducingdiffusion of claim 6, further comprising the process of forming anepitaxial layer over said spacer layer.
 8. The method of claim 6,wherein said spacer layer comprises epitaxial silicon and said dopantatoms include at least one of arsenic and antimony.
 9. The method ofclaim 6, wherein said forming and doping processes occur in asubstantially simultaneous manner,